You will need to draw a line graph with zero minutes on the left, and vertical lines at one minute intervals. Then you can draw in each car as it arrives and extend its line for the time taken.
But I suspect there is only one mechanic, so regardless of its arrival time, he can only start on a car when he's finished the one before. So a car can only be given a position in the queue. Instead of its actual arrival time, put the next car after the one before is finished.
but would you minus time elapsed from the wait time? as above min 1 first car takes 3 mins therfore second car waits 3 mins minute 2 car 1 then waits 2mins (3 less time lapsed for the first minute car 2 has waited already)
If the time taken to do the work is exactly the same then you might be able to work this out but when different faults take vastly different amounts of time I do not see how you can effectively do this. Unless I'm misreading what you want to do.
My friends have a garage and the repairs in some cases can take a day and in others maybe 10 minutes as it depends whats wrong. The best they can do is look at the car, see whats wrong and estimate how long it will take (based on experience) and then fit the repair in as best they can bearing in mind the number of mechanics available. And of course assuming no one is off in which case all timetables are out the window as they are short handed.
You are going to be hardpressed to come up with anything definitive as the number of variables are so great. Time taken by user to operate the pump, how much fuel is pumped, how long they take to pay etc. I wish you luck.
Draw a Horizontal line and divide into 60 units. i.e 1 min intervals. Draw vertical line and divide into any number to represent cars arriving.Put mark next to minute 1st car arrives and again when first car leaves join the marks. This will be his service time. Mark minute when 2nd car arrives and again when he leaves. This will be his service time. Where the two lines overlap will be the waiting time. e.g car 1 arrives at minute 1 and leaves at minute 4. Car 2 arrives at minute 3 and leaves at minute 6 overlap is from minute 3 till minute 4 therefor waiting time is 1 minute.